The present invention relates to a toner, to be used for electrophotograph or electrostatic printing. More particularly, the present invention is directed to provide a toner which is capable of exhibiting high transfer efficiency without fog and which comprises toner particles, containing a coloring agent, and external additives, and to provide an image forming apparatus using the same.
In the electrophotographic technology, after an electrostatic latent image formed on a photoreceptor having a photoconductive material is developed with toner particles, the toner image is fixed to a transfer medium such as a paper sheet with heat, pressure, and the like, thereby forming a copy or a printed matter.
It is a general way to add external additives to a toner in order to improve the properties of the toner. As the external additives, silicon dioxide, aluminum oxide, titanium oxide and the like are generally employed alone or in combination. In this case, in order to utilize the characteristics of the respective external additives, the combination use is common rather than the single use.
However, the characteristics of the toner depend on the properties of the toner particles such as the particle size and are therefore instable. For example, there is a distribution in charging amount so that a negatively charged toner inevitably includes positively charged toner particles.
In an image forming apparatus forming an image by means of negative charge reversal development, this leads a problem of increasing the fog because such toner particles adhere to non-image portions of the photoreceptor on which an electrostatic latent image is formed. Particularly, as the number of image formation increases, the toner contains particles of which original characteristics change. As a result, the amount of toner to be fog toner increases.
It is considered that a large amount of silica is added in order to maintain the characteristics of the toner to maintain the fluidity of the toner. Though the fluidity of the toner is improved, the fixing property of the toner is reduced.
Further, titania having positively chargeable property opposite to the polarity of the toner is added with silica to the toner in order to improve the negatively chargeable capacity of the toner, thereby improving the early rising property of negative charging of the toner. As the number of printed sheets increases, titania particles may be liberated from the surfaces of toner particles. To prevent the liberation of titania particles, the inventors of this invention have proposed a method of externally adding titania, of which work function is large, after externally adding silica, of which work function is small, to toner mother particles. In this method, the amount of silica to be added should be increased to correspond to the increase in amount of titania in order to compensate the amount of titania to be liberated from the surfaces of toner particles. However, as the amount of silica is increased, the fixing property of toner has a tendency to relatively decrease.
When the negatively chargeable property of the toner becomes too high, the density of printed images should be low. To prevent this, it is known to use titanium having relatively large primary particle diameter and relatively low electrical resistance in order to prevent the titania particles from being embedded into toner particles. However, as the number of image forming sheets increases, the liberation from the surfaces of toner mother particles may be caused, it is hardly exhibit the effects well.
Disclosed in JP (A) 2002-29730 is an electrophotographic toner in which fine silica particles are coated with hydroxide or oxide of one or more of titanium, tin, zirconium, and aluminum in a water system and further coated with alkoxysilane so as to prepare hydrophobic fine particles and the hydrophobic fine particles thus prepared are used as an external additive. Disclosed in JP(A) 2002-148848 is a toner in which silica particles are coated with titanium oxide so as to prepare titanium oxide particles containing silica and the titanium oxide particles thus prepared are used as an external additive. Though the addition of a predetermined amount of such an external additive can reduce the amount of positively charged toner particles, enough average charging amount can not obtained. Therefore, neither of these can satisfy both objects of improving the transfer efficiency and reducing the amount of reversely transferred toner particles.
It is an object of the present invention to provide a toner with which high transfer efficiency can be obtained with no or little fog at non-image portion.